The formation through which a wellbore is drilled exerts a variable force on the drill string at all times. This variable force is essentially due to the clockwise rotary motion of the bit, the weight applied to the drill bit and the strata of the formation. Formation is a general term used to define the material--namely rock, sand, shale, clay, etc.--that the wellbore will pass through in order to open a pathway or conduit to a producing formation. This variable force will result in a variable change in the direction of the wellbore.
The formation is generally layered by the action of nature over millions of years and is not necessarily level. The formation will have dips, defined as a change in direction of the layers of the formation, which can extend either upward or downward. As the drill bit moves into a dip or from one type of formation to another, the force on the drill bit will change and cause the drill bit to wander up, down, right or left. This wandering is the natural result of the reaction of the formation to the clockwise torque and forward drilling force exerted by the drill bit on the formation. Mathematically the result can be viewed as a simple vector cross product between the torque force and the drilling force or weight on bit. The cross product results in a component force towards the right of the drilling force. The industrial term given to this effect is "bit-walk" and many methods to control or re-direct "bit-walk" have been tried in the industry.
Bit-walk is predictable, but the magnitude and, frequently, the direction of bit-walk are generally unpredictable. Looking at the vector cross product model, it can be seen that as the drilling force or weight on bit is varied, the cross product varies. Or, as the RPM of the drill string is varied, the cross product varies. Or, as the formation changes, the cross product changes. In drilling a wellbore, all of these forces constantly vary; thus, the magnitude of bit-walk constantly changes. The industry has learned to control the effects of bit-walk in a vertical hole by varying the torque and weight on bit while drilling a vertical hole. However, in an inclined (non-vertical) hole bit-walk causes a number of problems.
By industry definition, once an inclined hole is established, the side of the wellbore nearest to true vertical is called the "low-side" of the hole. The opposite side of the hole is referred to as "high-side" and is used as a reference point throughout the wellbore drilling operation. The drilling force follows the longitudinal extension of the wellbore; thus, the drilling force is parallel to and spaced from the low-side of the hole. Since bit-walk is the result of applied torque and drilling force, then it can be anticipated that normal bit-walk will be to the right of the low-side of the hole. This definition applies in all wellbores.
In a vertical hole or slightly inclined hole, bit-walk may be controlled by developing as much rigidity as possible in the lower portion of the drill string near to the drill bit. This can be and generally is accomplished by using drill string components of high rigidity and weight (drill collars or heavy-weight drill pipe) and stabilizers. A stabilizer, well known in the industry, is a tubular member with a combination of radial blades, often having a helical configuration, circumferentially arranged around the tubular and extending beyond the outer diameter of the tubular. The extension of the stabilizer blades is limited to the diameter of the drill bit. Thus, the stabilizer will work in a stable hole; however, if the wellbore washes out (increases in diameter due to formation or other downhole mechanical or hydraulic effects) or where the lateral force exerted by the blades is less than the torque effect of the drill bit, then the stabilizer loses its effectiveness and bit-walk will occur. In a highly inclined or horizontal well, bit-walk becomes a major problem.
Very often the driller wishes to deviate the wellbore or control its direction to a given point within a producing formation. This operation is known as directional drilling. For example, a water injection well in an oil field is generally positioned at the edges of the field and at a low point in that field (or formation). A vertical wellbore will be established and the wellbore "kicked-off" from vertical so that an inclined (or even horizontal) wellbore results. It is now necessary to selectively guide the drill bit and string to the required point in the relevant formation. In order to achieve this objective, control of the wellbore is required in both the vertical plane (i.e. up and down) and in the horizontal plane (i.e., left and right).
At present, in order to deviate a hole left or right, the driller can choose from a series of special downhole tools or techniques. The industry often employs downhole motors and bent subs. More recently the steerable motor has become popular, although it uses similar precepts employed by the downhole motor and bent sub. Both of these tools act in a similar manner and both require that the drill string not be rotated in order to influence and control the wellbore direction.
A bent sub, a short tubular that has a slight bend to one side, is attached to the drill string, followed by a survey instrument, of which an MWD tool (Measurement While Drilling which passes wellbore directional information to the surface) is one generic type, followed by a downhole motor attached to the drill bit. The drill string is lowered into the wellbore and rotated until the MWD tool indicates that the leading edge of the drill bit is facing in the desired direction. Weight is applied to the bit through the drill collars and, by pumping drilling fluid through the drill string, the downhole motor rotates the bit. As the bit cuts the wellbore in the required inclination and direction, the drill string is advanced. When drilling with a bent sub and motor, after the correct inclination and direction are established, the entire string is tripped to the surface, the bottom hole assembly (bent sub, downhole motor and drill bit) replaced with a single drill bit, the string is then tripped into the wellbore, and regular drilling operations restarted. This procedure will be repeated if the direction of the wellbore is unsatisfactory.
The advantage of a steerable motor is that the assembly does not require tripping immediately after the correct inclination and direction are established; the motor can be retained and will drill as a conventional "rotary assembly". Whenever the assembly is tripped, a new bottom hole assembly will be configured which will, theoretically, allow continuation of the wellbore along the correct plane and at the correct angle from vertical.
It follows that the deeper or longer the wellbore, more time will be used in making a return trip whenever tools have to be changed. For example, the bent sub may not have enough angle which will always require a round trip.
One of the earlier inventions giving sufficient control to deviate and start an inclined hole from or control bit-walk in a vertical wellbore may be in found U.S. Pat. No. 3,561,549 entitled Slant Drilling Tools for Oil Wells by Garrison and Tschirky. Garrison et al. disclose an improvement in which a non-rotating sleeve having a plurality of fins (or wedges) on one side is placed immediately below a downhole motor in turn attached to a bit. This device acts in a similar manner to an offset packer and biases the downhole assembly away from the fins (or wedges). The device must be orientated like an offset packer before commencing drilling operations. Once the wellbore is established in the desired direction, the device must be taken out of service by a round trip out of and back into the wellbore. The disclosure discusses a second orientation device above the downhole motor. This device is more properly applied when starting an initial inclination or when correcting a vertical hole which has drifted from true vertical.
U.S. Pat. No. 4,220,213 by Hamilton discloses a Method and Apparatus for Self Orientating a Drill String while Drilling a Wellbore. The device consists of an offset mandrel with a rotatable tubular extending through the mandrel and a shoe, laterally attached to the outside of the mandrel, which slides along the wellbore. The offset mandrel is heavily weighted (by supplying sufficient material when manufacturing the mandrel) at 90 degrees to the "shoe." This tool is attached to the drill string immediately above the drill bit and the remaining drill string contains the usual downhole tools for weight, flexibility, control of inclination, wellbore surveying, etc. The heavily weighted portion of the Hamilton mandrel seeks the low-side of the hole, thus orientating the shoe to one side of the wellbore. The sliding shoe places a bias on the attached drill bit in a similar manner as does an offset packer or the Garrison et al. device.
The tool is designed to take advantage of gravity because the heavy side of the mandrel will seek the low-side of the hole. The shoe is attached to the mandrel on the side and one-quarter along the circumference. The device is designed to counteract the vector cross product of torque and drilling force which normally causes the bit to walk to the right. This means that a counter force must be applied that biases the bit to the left; thus, the normal position of the shoe is on the right. In using the tool, the weighted bottom seeks the low-side of the wellbore, the shoe rubs along the right side of the wellbore and the tubular rotates freely within the mandrel supplying drilling torque to the bit. The extension of the shoe beyond the bit circumference would be set by the size of the wellbore.
This tool is known to work: however, it suffers the same drawback as does the offset packer and the tool of Garrison et al., namely if the bit-walk forces change, then the tool must be changed or removed necessitating a round trip.
U.S. Pat. No. 4,638,873 to Welbom discloses a Direction and Angle Maintenance Tool and Method for Adjusting and Maintaining the Angle of a Directionally Drilled Borehole. This tool is essentially an improvement to the Hamilton device and operates in much the same manner. Welborn uses a spring-loaded shoe and a weighted heavy side which can accommodate a gauge insert held in place by a retaining bolt. Welborn explains that the lowside gauge insert will cause hole deviation (inclination) and the spring-loaded shoe will resist the tendency for bit-walk. He claims an improvement to the bearings within the mandrel, which reduces the tendency of the bearings to fail. The disclosure states that the gauge insert is chosen to obtain a particular change in inclination and that the shoe may be used (or left off) to correct bit-walk to the right. If a change in bit-walk rate occurs or if the bit tends to move to the left, then this tool, like the other tools described, must be withdrawn. This necessitates a round trip.
Thus, the prior art can correct bit-walk in a wellbore. However, if changes in the forces that cause bit-walk occur while drilling, all the prior art tools must be withdrawn in order to correct the direction of the wellbore. The absolute requirement for tool withdrawal means that a round trip must be performed. This results in a compromise of safety and a large expenditure of time and money. The industry needs a true lefl/right downhole tool that can remain in place on the downhole assembly and have its effect switched from the surface. That is, a tool that will cause the wellbore to turn either to the right or to the left whenever required.